A data transmission system is generally employed for transmitting data from the transmitter to the receiver where a carrier wave is modulated by a sending data signal.
This data transmission system comprises transmitting terminal equipment, a modulator, a demodulator and receiving terminal equipment. The transmitting terminal equipment is connected to the modulator, while the receiving terminal equipment is connected to the demodulator, and the modulator and demodulator are respectively connected by the telephone lines.
The signal sending and receiving methods between the equipment in the data transmission system having the above structure are specified in CCITT (THE INTERNATIONAL TELEGRAPH AND TELEPHONE CONSULTATIVE COMMITTEE) Recommendation V. 29.
According to this recommendation, when the transmitting terminal equipment issues a transmission request signal to the modulator, the demodulator should, in response to such a request signal, send a carrier detection signal to indicate that data is to be sent from the transmitter to the receiving terminal equipment.
In one prior demodulator, a carrier detection signal provides an output in accordance with the carrier extracted from the receiving signal in the demodulator.
On the other hand, it is proposed that such a data transmission system be subject to the condition that the carrier is always transmitted. Therefore such a data transmission system is required to accurately discriminate between the period where the data corresponding to predetermined information which must be transmitted to the receiving terminal equipment from the transmitting terminal equipment exists and the period where such data does not exist. But the prior system only detects the carrier signal and therefore it cannot detect the effective data period where data to be transmitted exists.